change return type of cacheKey() to non-nullable

lib/models/ente_file.dart

@@ -4,5 +4,5 @@
 abstract class EnteFile {
   // returns cacheKey which should be used while caching entry related to
   // this file.
-  String? cacheKey();
+  String cacheKey();
 }

lib/models/file.dart

@@ -275,9 +275,8 @@ class File extends EnteFile {
   }
 
   @override
-  //add nullable return type when migrating to null safety
   String cacheKey() {
     // todo: Neeraj: 19thJuly'22: evaluate and add fileHash as the key?
-    return localID ?? uploadedFileID?.toString() ?? generatedID?.toString();
+    return localID ?? uploadedFileID?.toString() ?? generatedID.toString();
   }
 }